Project Summary/Abstract In our application, Aligning Resources for the Care of Homeless Veterans (ARCH) we propose studying ways to best organize and deliver primary care for homeless Veterans. We will assess 4 different adaptations of the PACT primary care model in a multi-center, randomized-controlled 2x2 comparative effectiveness study to determine optimal care approaches measured by reductions in avoidable emergency departments and preventable hospitalizations, increased patient satisfaction, improved chronic disease management and earlier transitions out of homelessness. Findings from this study will substantively add to our understanding and advance the fields of health seeking behavior, care of vulnerable/high-risk veteran populations and clinical systems design. Second, it reflects a true field-based study employing study sites in a methodologically rigorous manner that both advances our knowledge in this area as well as identifies optimal and feasible approaches to patient care within our current VHA system. Finally, it will help inform pressing policy issues relevant to two identified T-21 priority areas: Ending Veteran Homelessness in 5 Years and Transforming to a Patient Centered Primary Care model.